1. Unit 4: Energy and Momentum
Chapter 10 Energy Flow and Power
10.1 Efficiency
10.2 Energy and Power
10.3 Energy Flow in Systems

2. Chapter 10 Objectives
Give an example of a process and the efficiency of a process.
Calculate the efficiency of a mechanical system from energy and work.
Give examples applying the concept of efficiency to technological, natural and biological systems.
Calculate power in technological, natural, and biological systems.
Evaluate power requirements from considerations of force, mass, speed, and energy.
Sketch an energy flow diagram of a technological, natural, or biological system.

3. Chapter 10 Vocabulary Terms
efficiency
input
output
energy
6 energy forms
Power
horsepower
watt
cycle
food chain
joule
energy conversion
steady state
Energy flow diagram
Thermodynamics
Entropy

4. Energy Flow and Systems
Mr. Pike BHS Physics Mr. Yates
1/1/2009
Chapter 10
Energy Flow and Systems
Thermodynamics
Technology and thermodynamics
Thermodynamics is the physics of heat. Since so much of our technology depends on heat, it is important to understand thermodynamics.

5. Energy Flow and Systems
Mr. Pike BHS Physics Mr. Yates
1/1/2009
Chapter 10
Energy Flow and Systems
Laws of Thermodynamics
The law of conservation of energy is also called the first law of
thermodynamics. It says that energy cannot be created or destroyed, only converted from one form into another.
The second law of thermodynamics says that when
work is done by heat flowing, the output work is
always less than the amount of heat that flows.
A car engine is a good example.

6. Mr. Pike BHS Physics Mr. YATES
1/1/2009
The 1st law is a STATEMENT OF ENERGY CONVERSION because it states that the amount of energy never changes
The 2nd law is also known as the law of increasing entropy because it states that the QUALITY of that energy decreases over time…100% of energy cannot be transformed to work. Usable energy is inevitably used. In the process, usable energy is converted into unusable energy. Thus, usable energy is irretrievably lost in the form of unusable energy (usually heat).

7. ENTROPY The measure of disorder in a system
Mr. Pike BHS Physics Mr. YATES
1/1/2009
ENTROPY
The measure of disorder in a system
Better put: It is the measure of unusable energy in a system
Useful work must be derived from the energy that flows from the high level to the low level. As things move toward that lower energy level, there is an increase in unusable energy (energy is converted to an unusable form)…an increase in entropy in the system
As usable energy decreases and unusable energy increases, "entropy" increases

8. Mr. Pike BHS Physics Mr. YATES
1/1/2009
1st and 2nd laws in action
You eat, you gain energy that existed from somewhere else
You use some of that energy. Most is given off as heat. That heat energy is still there, you just cannot use it again. It becomes unusable energy

9. Efficiency
According to the law of conservation of energy, energy cannot ever be truly lost, so the total efficiency of any process is 100%.
The work output is reduced by the work that is converted to heat, resulting in lower efficiency.

10. Efficiency Efficiency is defined for a process.
A process is any activity that changes things and can be described in terms of input and output.
The efficiency of a process is the ratio of output to input.

11. Efficiency
Efficiency can also mean the ratio of energy output divided by energy input.
Energy output (J)
e = Eo Ei
Efficiency
Energy input (J)

12. Efficiency in natural systems
Energy drives all the processes in nature, from winds in the atmosphere to nuclear reactions occurring in the cores of stars.
In the environment, efficiency is interpreted as the fraction of energy that goes into a particular process.

13. Efficiency in biological systems
In terms of output work, the energy efficiency of living things is typically very low.
Almost all of the energy in the food you eat becomes heat and waste products; very little becomes physical work.

15. Efficiency in biological systems
Think of time as an arrow pointing from the past into the future.
All processes move in the direction of the arrow, and never go backward.

16. Efficiency in biological systems
Since processes in the universe always lose a little energy to friction, time cannot run backward. Those processes will never spontaneously gain that energy back!
If you study physics further, this idea connecting energy and time has many other implications.

17. Calculate efficiency
A 12-gram paper airplane is launched at a speed of 6.5 m/sec with a rubber band.
The rubber band is stretched with a force of 10 N for a distance of 15 cm.
Calculate the efficiency of the process of launching the plane.
1) You are asked for the efficiency.
2) You are given the input force and distance and the output mass and speed.
3) Efficiency is output energy divided by input energy.
The input energy is work = F x d.
The output energy Ek = 1/2 mv2.
4) Solve:
ε = (0.5)(0.012 kg)(6.5 m/sec)2 / (10 N)(0.15 m)
= 0.26 or 26%

18. Energy Flow and Systems
Mr. Pike BHS Physics Mr. YATES
1/1/2009
Chapter 10
Energy Flow and Systems
Energy and systems
Energy as nature’s“ money”
Energy exists in many forms and can be changed from one form to another.
You can think of energy as nature’s money. It is spent and saved in a number
of different ways. You can use energy to buy speed, height, temperature,
mass, and other things.
Can you predict how fast the green ball will
be launched if the only source of energy is the falling blue ball?

19. Energy Flow and Systems
Mr. Pike BHS Physics Mr. YATES
1/1/2009
Chapter 10
Energy Flow and Systems
Energy exists in many different forms
Mechanical energy is the energy an object has due to its motion or position: Potential and Kinetic
Radiant energy is also known as electromagnetic energy.
Electrical energy is carried by the flow of electric current.
Chemical energy is energy stored in the bonds that join atoms.
Nuclear energy results from splitting up large atoms (like uranium) or combining small atoms (like hydrogen) to form larger ones.
Thermal energy takes the form of heat.
The pressure in a fluid or gas is a form of energy.

20. Energy flow in systems
Energy flows almost always involve energy conversions.
To understanding an energy flow:
Write down the forms that the energy takes.
Diagram the flow of energy from start to finish for all the important processes that take place in the system.
Try to estimate how much energy is involved and what are the efficiencies of each energy conversion.

21. Energy flow in systems
A pendulum is a system in which a mass swings back and forth on a string.
There are 3 chief forms of energy: potential energy, kinetic energy, and heat loss from friction.

22. Energy flow in human technology
The energy flow in technology can usually be broken down into four types of processes:
Storage ex. batteries, springs, height, pressure
Conversion ex. a pump converting mechanical energy to fluid energy
Transmission ex. through wires, tubes, gears, levers
Output ex. heat, light, electricity

23. Energy flow
The energy flow diagram for a rechargeable electric drill shows losses to heat or friction at each step.

24. Energy flow in natural systems
The energy flows in technology tend to start and stop.
Many of the energy flows in nature occur in cycles.
Water is a good example.

26. Energy flow in natural systems
A food chain is a series of processes through which energy and nutrients are transferred between living things.
A food chain is like one strand in a food web.
A food web connects all the producers and consumers of energy in an ecosystem.

27. Energy flow in natural systems
The energy pyramid is a good way to show how energy moves through an ecosystem.

28. Energy Flow in Systems Key Question: Where did the energy go?
*Students read Section BEFORE Investigation 11.3

29. Application: Energy from Ocean Tides

30. Energy and Power Key Question: How powerful are you?
*Students read Section AFTER Investigation 11.2

31. Energy and Power It makes a difference how fast you do work.
Suppose you drag a box with a force
of 100 newtons for 10 meters in 10 seconds. You do 1,000 joules of work. Your
friend drags a similar box and takes 60 seconds. You both do the same amount of
work because the force and distance are the same. But something is different. You
did the work in 10 seconds and your friend took six times longer.

32. Power A unit of power is called a watt.
Another unit more familiar to you is horsepower.
One horsepower (the ave power output of a horse) is equal to 746 watts.

33. Power
Power is the rate at which energy converts or work is done.
Change in work
or energy (J)
Power (W)
P = E t
Change in time (sec)

34. Calculate power A 70 kg person goes up stairs 5 m high in 30 sec.
(1) You are asked for power.
(2) You are given mass, distance, and time.
(3) Relationships that apply:
Ep = mgh P = E/t
(4) Solve:
Ep = (70 kg)(9.8 N/kg)(5 m) = 3430 J
P = (3,430 J)/(30 sec) = 114 watts
(a) 114 W
(b) This is a little more power than a 100-watt light bulb.
Many human activities use power comparable to a light bulb.
A 70 kg person goes up stairs 5 m high in 30 sec.
a) How much power does the person need to use?
b) Compare the power used with a 100-watt light bulb.

35. 10.2 Power Three ways to look at power Power in flowing energy
1/1/2009
Chapter 10
Energy Flow and Systems
Power in flowing energy
Three ways to look at power
The first kind is work being done by a force. P = Fd/t
The second situation is energy flowing from one place to another, such as electrical energy flowing through wires.
The third situation is when energy is converted from one form to another. P = J/t
10.2 Power

36. Power
Another way to express power is as a multiple of force and it's velocity, if the velocity and force are both vectors in the same direction.
Power (W)
P = F . v
Velocity (m/sec)
Force (N)

37. Power in human technology
You probably use technology with a wide range of power every day.
Machines are designed to use the appropriate amount of power to create enough force to do work they are designed to do.

38. Estimate power A fan uses a rotating blade to move air.
How much power is used by a fan that moves 2 m3 of air each second at a speed of 3 m/sec? Assume air is initially at rest and has a density of 1 kg/m3.
Fans are inefficient; assume an efficiency of 10 %.
(1) You are asked for power.
(2) You are given volume, density, speed, and time.
(3) Relationships that apply:
ρ = m/V, Ek = 1/2 mv2, P = E/t
(4) Solve:
m = ρV = (1 kg/m3)(2 m3) = 2 kg
Ek = (0.5)(2 kg)(3 m/sec)2 = 9 J
At an efficiency of 10 percent, it takes 90 joules of input power to make 9J of output energy.
P = (90 J)/(1 sec) = 90 watts

39. Power in natural systems
Natural systems exhibit a much greater range of power than human technology
The sun has a total power output of 3.8 × W.
The power received from the sun is what drives the weather on Earth.

40. Power in biological systems
200 years ago, a person’s own muscles and those of their horses were all anyone had for power.
Today, the average lawn mower has a power of 2,500 watts—the equivalent power of three horses plus three people.
Most of the power output of animals takes the form of heat.
The output power from plants is input power for animals.

41. Estimate power An average diet includes 2,500 food calories/day.
Calculate the average power this represents in watts over a 24-hour period.
One food calorie equals 4,187 joules.
(1) You are asked for power.
(2) You are given the energy input in food calories and the time.
(3) Relationships that apply:
1 food calorie = 4,187 J, P = E/t
(4) Solve:
E = (2,500 cal)(4,187 J/cal)
= 10,467,500 J
There are 60 x 60 x 24 =
86,400 seconds in a day.
P = (10,467,500 J) / (86,400 sec) = 121 watts
This is a bit more than the power used by a 100-watt light bulb.